This invention relates to the problem of distortion in amplified signals produced by a field effect transistor (FET) due to changes in the bias current in the FET caused by fluctuations in the operating voltage supply and, more particularly, to one solution to this problem employing the use of a transistor circuit whose impedance characteristic compensates for such bias current changes.
Recently, a field effect transistor of the type having triode-type dynamic characteristics has been developed. This type of FET offers many advantageous features, such as a very small output resistance, the avoidance of drain current saturation with an increase in drain voltage and superior voltage-current characteristic linearity. Because of these and additional features, the FET having triode-type dynamic characteristics finds ready application as an amplifier with high signal fidelity.
However, it has been found that this type of FET is particularly sensitive to fluctuations in the operating potentials supplied thereto. That is, the drain bias current will vary as the power supply voltages applied to the FET vary. As a result thereof, distortion is introduced into the amplified signal produced by the FET. One proposal for avoiding this type of distortion is disclosed in copending U.S. Application Ser. No. 508,836, filed Sept. 24, 1974, and assigned to the assignee of the instant invention.
While this earlier proposal is satisfactory for many applications, a problem may occur when the FET which is used in one circuit is not identical to the FET which is used in another, although substantially identical, circuit. Hence, if the FET parameters vary from devices to device, and in a random manner, the DC bias current to the drain may not be fully compensated when the power supply is subjected to fluctuations.
The present invention proceeds upon the recognition that in FET's having triode-type dynamic characteristics, the pinch-off voltage V.sub.P can be a random value and the amplification factor .mu. likewise can be a random or arbitrary value; but these physical quantities are related by EQU V.sub.P.sup.. .mu. = constant
The graphical representation of this relation is that of a hyperbola. This relationship is turned to account to compensate for changes in the DC bias current at the FET drain attributed to operating voltage fluctuations even though the particular FET which is used in one circuit has different characteristics than an FET which may be used in another circuit.